1. Field of Application
The present invention relates to a gas concentration detection apparatus, and in particular to a gas concentration detection apparatus for detecting concentrations of gaseous components such as oxygen, etc., in an exhaust gas of an internal combustion engine.
2. Description of the Prior Art
Types of gas concentration detection apparatus are known that are utilized as an air/fuel ratio sensor (generally abbreviated to A/F sensor) for the engine of a vehicle, with such a sensor detecting the concentration of oxygen in the exhaust gas from the engine. In particular, a planar type of A/F sensor is known, having a sensor element containing a layer of solid electrolyte, with a pair of electrodes mounted on that layer. When a voltage is applied between the electrodes, a current flows through them at a level determined by the concentration of oxygen in the exhaust gas. The air/fuel ratio of the exhaust gas is measured based on the level of that current.
With such an A/F sensor it is necessary for the sensor to be in an activated condition in order to accurately detect the oxygen concentration. The impedance of the sensor element (i.e., at any specific value of AC frequency) varies in accordance with the activation status of the sensor element, and hence the impedance can be measured to evaluate the activation status of the element. One method known in the prior art for measuring the oxygen concentration and the sensor element impedance concurrently, is to apply an AC voltage across the sensor element electrodes and to detect the amplitude of a resultant AC current that flows through the sensor. Such a method is described for example in Japanese patent application second publication No. 4-24657 (referred to in the following as reference document 1). An example of a circuit configuration for implementing such a detection method is shown in FIG. 10.
In FIG. 10, one terminal of a sensor element 60 is connected in series with a current measurement resistor 63, an oscillator 62 and a reference voltage source 61 (with the reference voltage source 61 being connected between the oscillator 62 and ground potential as shown), while the other terminal of the sensor element 60 is connected to ground potential. The input terminals of a differential amplifier 65 are connected across the terminals of the differential amplifier 65, and the output signal of the differential amplifier 65 is transferred through a LPF 66 and through a HPF 67. With the sensor element 60 exposed to an exhaust gas, when an AC voltage and superimposed DC voltage are applied to the sensor element 60 by the oscillator 62 and reference voltage source 61 and a resultant sensor current flows in the sensor element 60, the current contains a component (DC component) at a level determined by the oxygen concentration in the exhaust gas and a component (AC component) having an amplitude determined by the impedance of the sensor element 60. A differential voltage signal varying in proportion to the sensor current appears between the terminals of the current measurement resistor 63, with that differential voltage signal being supplied to the differential amplifier 65, to be amplified and converted to a voltage signal which varies with respect to the system ground potential.
A DC voltage signal component (corresponding to the DC component of the sensor current whose level is determined by the oxygen concentration) is extracted from the output of the differential amplifier 65 by the LPF 66, while an AC voltage signal component (corresponding to the AC component of the sensor current, whose amplitude is determined by the sensor impedance) is extracted by the sensor element 60 from the output of the differential amplifier 65. That AC voltage signal is rectified by a rectifier circuit 68, to obtain a voltage signal that varies in level in accordance with the impedance of the sensor element 60.
These output (analog) voltage signals from the LPF 66 and rectifier circuit 68 are inputted to a calculation apparatus (digital processing apparatus) such as a microcomputer, with the signals being converted to digital form in the calculation apparatus or before being inputted to the calculation apparatus. The calculation apparatus calculates the respective values of the air/fuel ratio and the sensor element impedance based on these input signals.
In general, there will be a large difference between the respective levels of the sensor current component that varies in accordance with the oxygen concentration and the sensor current component that varies in accordance with the sensor element impedance. As a result, the voltage signal component (inputted from the current measurement resistor 63 to the differential amplifier 65) representing the air/fuel ratio will be substantially smaller (in some cases, by an order of magnitude) than the voltage signal component that varies in accordance with the sensor element impedance.
Hence if for example the degree of amplification is determined based on the expected range of variation of the voltage signal component corresponding to the air/fuel ratio, the variations in amplitude of the voltage signal component corresponding to the sensor element impedance may exceed the range of values that can be amplified by the differential amplifier 65. This will not only result in a lowered accuracy of detecting the sensor element impedance, but will also result in a lowering of the accuracy of detecting the air/fuel ratio. That is to say, although the air/fuel ratio detection signal is extracted by averaging the output signal from the differential amplifier 65 using a LPF 66, if the upper limit of the range of amplification of the differential amplifier 65 is exceeded, the resultant signal from the LPF 66 will not accurately represent the average value, causing errors in the measured air/fuel ratio.
Conversely if the degree of amplification were to be predetermined based on the expected range of variation of the amplitude of the AC voltage signal component that varies in accordance with the sensor element impedance, then it would not be possible for the differential amplifier 65 alone to apply sufficient amplification to the air/fuel ratio detection component. Hence it would be necessary to utilize an additional amplifier stage to further amplify the voltage signal component used for air/fuel ratio detection. However this will result in the problem of an increase in amplifier offset voltage, which could cause a lowering of accuracy of air/fuel ratio detection.